1. Field of the Invention
The present invention relates to dichroic antennas. More specifically, the present invention relates to a novel and improved technique for manufacturing frequency-selective surface antennas, particularly of the type having a frequency selective- surface formed on the reflecting surface of a parabolic lens reflector for dichroic antenna applications.
2. Description of the Related Art
In certain antenna designs, particularly in high frequency aerospace applications, it is desirable to fabricate high gain antennas which will receive signals of differing frequency bands. Antennas having a frequency-selective surface can provide the necessary high gain, multi-frequency capabilities that are often desired in microwave tracking and radar systems. One type of antenna utilizing the frequency-selective surface for multi-frequency applications is one that uses a dichroic parabolic reflector.
One particular type of dichroic antenna includes a parabolic reflector antenna mounted in front of a slotted array antenna. In this configuration the parabolic reflector antenna is designed to operate at higher frequencies than the slotted array antenna. The parabolic reflector antenna portion of the dichroic antenna includes a feed horn positioned at the focus of the parabolic reflector and associated waveguide and support structure. The parabolic reflector typically has a metalized surface upon which resonant dipoles, particularly cross dipoles, are formed therein. The parabolic reflector is usually formed from a flat substrate of low dielectric constant material which has a metalized front, or reflecting surface, in which dipoles are formed. The dipoles act as reflectors, particularly at frequencies within the resonant frequency band, while the structure acts transparent to lower frequencies. The dipole reflected signal is received by the feed horn positioned at the focal point of the parabolic reflector. Another type of dichroic antenna is disclosed in the article entitled "Frequency-Selective Surfaces For Multiple-Frequency Antennas," by G. H. Schennwm, Microwave Journal (May, 1973).
Attempts have been made at fabricating high performance dichroic antennas which utilize a parabolic lens reflector having dipole elements formed upon the parabolic reflector surface. Using prior known fabrication techniques, construction of a dichroic parabolic reflector have resulted in structures in which signals at frequencies outside of the dipole resonant frequency band are basically unaffected as they pass through the parabolic reflector structure. This characteristic greatly affects the overall performance of the antenna, especially the sensitivity of the lower frequency band antenna positioned behind the parabolic reflector antenna.
One technique for fabricating a parabolic reflector having a frequency-selective surface is by mounting upon a low dielectric constant foam body, a parabolic shaped substrate having a dipole grid pattern formed upon a front surface. The substrate is typically a sheet of low dielectric plastic or polyethylene material, such as a material sold under the trade name DUROID, having copper clad parallel planer front and back surface. The copper cladding is typically in the range of approximately 0.0010 to 0.0015 inch thick. The copper clad substrate is photoengraved to define, upon later etching, the desired dipole grid pattern in the front surface metallization. The substrate is restrained in a holding fixture, heated if necessary, and then pressed between matching male and female paraboloid shaped mandrels to give the substrate the desired parabolic shape.
After shaping, the back surface metallization is removed, typically by chemical etching techniques. Prior to etching the back surface, a protective layer may be applied to the front surface metallization to prevent chemical attack thereto. It is desired to leave the front surface metallization intact to enhance rigidity and shape retention of the formed substrate. However, in some cases the front surface metallization may be removed at the same time as the back surface metallization.
After etching, an adhesive material is then deposited upon the exposed back surface of the substrate. The substrate is then positioned for mounting the adhesive-backed substrate back surface upon a mating parabolic surface of a low dielectric constant foam body formed from a material such as polyurethane. One method for mounting the substrate upon the foam body is by stamping or pressure-forming the substrate upon the parabolic surface of the foam body. The adhesive is then cured to secure the substrate upon the foam body. The front surface is then chemically etched such that the desired dipole grid pattern is the only remaining metallization on the front surface.
Although fabrication by the just-described technique will achieve a functional parabolic reflector, use of such a reflector in a dichroic antenna can result in substantially less than optimal performance characteristics. For example, the use of a substrate for carrying the dipole grid pattern can itself provide attenuation of signals passing therethrough.
Another problem arises after the flat substrate has been formed into a parabolic shape. When the front surface metallization is etched leaving a dipole grid pattern. The once continuous metallization layer is therefor disrupted. With the metallization layer disrupted, the substrate tends to "pull back" a minute distance towards its original planar form. Even when the front surface etching occurs before the adherence to the foam backing, the substrate still tends to pull back into its original planar form. Another hindrance in achieving optimum antenna performance is a result of the defining of the individual dipole shapes. Defining the shape of the dipoles when the substrate is planar does not result in ideal dipole shapes for parabolic reflector form, especially for dipoles positioned adjacent the upper rim of the parabolic contour. In high frequency applications, such as in the millimeter wave frequency band and especially in the M frequency band, reflector parabolic shape and individual dipole shape become more critical than at lower frequencies. The "pulling back" of the substrate can disrupt the desired parabolic reflector shape. Furthermore, improperly shaped dipoles can seriously and adversely affect the antenna performance characteristics.
In the construction of parabolic lens reflectors, one commonly used material for the foam body is a material sold under the trademark ROHACELL. ROHACELL is a foam material having a low dielectric constant, typically in the range of approximately 1.05-1.15. Using a material with such a low dielectric constant minimally attenuates the portion of the signal outside of the dipole resonant frequency band as it passes through the parabolic lens reflector structure. However, foam bodies formed of ROHACELL and other types of low dielectric constant materials are typically of an insufficient stiffness to support direct dipole metallization in the required highly accurate parabolic contour. Various direct application techniques have been previously attempted but are undesirable because the required precise parabolic shape cannot be maintained.
Another method of forming reflective dipoles upon parabolic reflector surfaces utilizes direct transfer techniques. Some of these techniques, for example, utilize the transfer of a molded metal layer to a supporting base of some type. All of the transfer techniques use injected or flexible materials for either the transfer medium or the transferred to medium. The transfer techniques can affect the required precise parabolic surface curvature upon which the dipole grid pattern is formed.
It is, therefore, an object of the present invention to provide a novel and improved method for fabricating a parabolic lens reflector for implementing in a parabolic reflector antenna portion of a dichroic antenna.
It is yet another object of the present invention to provide a method for fabricating a dichroic parabolic lens reflector in which the fabrication methods are readily adapted to high volume manufacturing processes thus enabling production of highly reliable, accurate dichroic parabolic lens reflectors.